The present invention relates to a magnetic recording/reproducing circuit system wherein a video recording signal and a voice recording signal are supplied to a magnetic head to be recorded as superimposed on a magnetic recording medium and also to be reproduced from the recording medium.
A most ordinary apparatus for recording and reproducing a video signal and a voice signal is a video tape recorder (which will be abbreviated to the VTR, hereinafter).
In the VTR, a brightness signal in the video signal is FM-modulated at an FM modulator, a color signal therein is frequency-converted in its low band, and the brightness and color signals are then mixed together to form a recording signal. The recording signal is alternately supplied to two video heads through a recording amplifier and a rotary transformer to be recorded on video tape being fed as contacted with the video heads.
The recording amplifier is previously adjusted so that a recording current becomes its optimum value (at which the maximum reproduction output is obtained).
Meanwhile, a voice FM signal is subjected to a processing that is substantially the same as the brightness signal of the above video signal. In a high tone-quality VHS type VTR, in particular, the voice signal is recorded in a deep layer of the magnetic tape earlier than the video signal. In other words, the video signal is recorded as superimposed on a voice-signal recording part of the deep layer while erasing the already recorded voice-signal part of the surface layer of the magnetic tape.
The video and voice signals recorded in the video tape are alternately again reproduced by two video heads. Since the reproduced low-frequency carrier FM signal produces an output voltage as low as several 100 .mu.V, these signal are amplified at respective reproducing amplifiers. With regard to the brighgtness signal in the amplified video reproduction signal, the reproduction signal is passed through a high-pass filter to remove low-frequency converted color signal components from the brightness signal, through a drop-out compensation circuit to erase a signal defect accidentally occurred at the time of its recording and reproducing, and through a limiter to remove level variation components therefrom, and then sent to an FM demodulator for its demodulation.
When the recording signal is recorded on the video tape through the magnetic head, an anti-magnetizing force H.sub.c of magnetic material of the video tape varies with temperature, even when the recording amplifier is previously set so that the recording current has its optimum value as mentioned above. For this reason, a variation in the surrounding temperature also causes a variation of the optimum value of the recording current, which results in that, even when recording is carried out with the recording current having a constant amplitude, the reproduction-voice output signal voltage inevitably varies with temperature.
FIG. 1 is a graph showing (in relative-value scales) a reproduction video FM signal voltage characteristic (molar characteristic) to the video recording current value with respect to different temperature parameters. In the drawing, measurement points at ambient temperatures of 0.degree., 25.degree. and 50.degree. C. are denoted by .largecircle., x and .DELTA. respectively.
It will be appreciated from the graph that, for any of these temperatures, as the recording current is gradually increased from zero, the reproduction video FM signal voltage increases and reaches at its maximum level and then gradually decreases. That is, a current value I.sub.o of the reproduction signal providing the maximum output voltage corresponds to the optimum value of the video recording current for the associated temperature. As will be seen from FIG. 1, the optimum value (I.sub.L) of the video recording current at a low temperature is larger than that at a normal temperature and the optimum value (I.sub.H) of the video recording current at a high temperature is smaller than that at the normal temperature.
This is because the anti-magnetizing force H.sub.c of the magnetic tape drops with the increased temperature. It will be noted from the above consideration that a rise in the surrounding temperature causes the optimum value of the video recording current to be shifted to its smaller value, while a drop in the surrounding temperature causes the optimum value of the video recording current to be shifted to its larger value.
As mentioned above, in the high tone quality VHS type VTR, the FM voice signal is recorded in the deep layer of the video tape whereas the video signal is recorded in the surface layer thereof. Since the video signal is recorded while erasing the surface layer having the voice FM signal already recorded thereon, the reproduction output voltage of the voice FM signal is dependent on the recording level of the video signal. In other words, when the recording level of the video signal becomes high, the reproduction output voltage of the voice FM signal becomes small, whereas, when the recording level of the video signal becomes small, the reproduction output voltage of the voice FM signal becomes large.
FIG. 2 is a graph showing (in relative-value scales) a reproduction voice FM signal output voltage characteristic to the video recording current. It will be seen from FIG. 2 that, as the video recording current increases, the reproduction voice FM signal output voltage decreases.
As will be understood from FIGS. 1 and 2, even when the video recording current is set to have its optimum value I.sub.o while the reproduction video signal output is set to have its reference value (0 dB) at the normal temperature, the recording current value becomes low at the low temperature while becomes too high at the high temperature. Accordingly, even when the reproduction voice FM signal output level is set to have its optimum V.sub.o (0 dB), the reproduction voice FM signal output level becomes as too high as V.sub.L at the low temperature while becomes as too low as V.sub.E, as shown in FIG. 2.
Generally speaking, when the level of an output signal reproduced by a magnetic head is reduced, a ratio of the signal output to preamplifier noise or tape noise (S/N ratio) becomes small so that the tone quality is deteriorated, thus disabling high tone-quality reproduction.